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Use this version of the COLLECT FROM [WHERE] function for complex nested selections in which it is necessary to use an alias.

Syntax

 COLLECT entity | attribute FROM collection NAMED alias [WHERE expression]

• entity or attribute - Entity or attribute to collect. This should contain the alias, e.g. alias.Name.
• collection - A collection of entity instances.
• alias - A name for the collection.
• expression - Boolean expression that represents the criterion the instance has to meet.

Return type

• collection of entity instances
• collection of attribute values

Examples

Suppose the following model with only instances of entity Person. Person.hasChildren is a relation from Person to Person.

To retrieve all the parent names, we use this expression:

COLLECT Parent.name 
FROM
  COLLECT Person 
  FROM ALL Person 
  WHERE (Person.hasChildren != ?) NAMED Parent

This expression results in “Rick”, “Julia” and “Joan”.

To retrieve all children names, we use this expression:

COLLECT Child
FROM (
  COLLECT Person.hasChildren 
  FROM ALL Person 
  WHERE (Person.hasChildren != ?)) NAMED Child

This expression results in a collection of three instances of Person (i.e Person_1, Person_3 and Person_5, with names Kim, Bob and Sam).

To retrieve all children younger than 15, we use this expression:

COLLECT Child 
FROM (
  COLLECT Person.hasChildren
  FROM ALL Person 
  WHERE (Person.hasChildren != ?)) NAMED Child
WHERE (Child.Age < 15)

This expression results in a collection of two instances of Person (i.e Person_3 and Person_5, with names Bob and Sam).

To retrieve the children’s names with a parent older than 40 years:

COLLECT Child.Name 
FROM (
  COLLECT Person.hasChildren 
  FROM ALL Person 
  WHERE (Person.hasChildren != ? AND Person.Age > 40)) NAMED Child  

This expression results in a collection with only the name “Kim”.

You can make it as complex as you like, for example:

COLLECT Child.Name 
FROM (
  COLLECT Parent.hasChildren
  FROM (
    COLLECT Person
    FROM ALL Person 
    WHERE (Person.hasChildren != ?)) NAMED Parent
  WHERE (Parent.Age > 40 )
) NAMED Child 
WHERE (Child.Age < 18) 

This expression results also in a collection with only the name “Kim”.

thisPerson := Person ;
 
COLLECT Person
    FROM ALL Person 
    WHERE (Person != thisPerson) 

This function determines the size of a collection.

Syntax

SIZE ( collection )

• collection - A collection of attribute or entity instances. This can be an expression or a relation attribute for instance.

Return type

• integer

Example

Suppose you have a Parent and a Child entity, where Parent has a multivalued relation with Child via the relation Parent.has_Children. With this model the following instances are created:

Then:

• if Parent_1 is active, SIZE ( Parent.has_Children ) = 3
• if Parent_2 is active, SIZE ( Parent.has_Children ) = 1
• without an active Parent instance, SIZE ( Parent.has_Children ) results in an error
• if Child_1 is active, SIZE ( Child.hobbies ) = 2
• if Child_3 is active, SIZE ( Child.hobbies ) = 3
• if Child_4 is active, SIZE ( Child.hobbies ) = 1
• SIZE ( ? ) = 0

This function determines the intersection of two collections. It returns a collection containing the items that are present in both specified collections.

Syntax

INTERSECTION ( collection1 , collection2 )

• collection1, collection2 - Collections to be intersected. These collections must be of the same base type.

Return type

• collection

Examples

Suppose the following model. Entity Teacher has a multivalued relation with entity Child via the relation Teacher.teaches_Children.

• INTERSECTION ( Teacher[Teacher_1].teaches_Children , Teacher[Teacher_2].teaches_Children ) results in a collection of Child instances Child_1 and Child_3
• INTERSECTION ( Teacher[Teacher_1].teaches_Children.name , Teacher[Teacher_2].teaches_Children.name ) = “Kim”, “Bob”
• INTERSECTION ( Child[Child_1].hobbies , Child[Child_3].hobbies ) = “Reading”
• INTERSECTION ( Child[Child_2].hobbies , Child[Child_3].hobbies ) results in an empty list

DIFFERENCE

DIFFERENCE ( collection1 , collection2 )

SYMMETRIC_DIFFERENCE ( collection1 , collection2 )

Collections vs. lists

UNIQUE, SUBSET, UNION and INTERSECTION return a collection of data, i.e. double entries are removed. However, COLLECT statements, + and - return a list of data. You have to be aware of this when using the SIZE or UNPACK function.

Example

Statuses = ["a","b","c","d","a","b","c","d"].

Result

The expression "SIZE ( COLLECT Status FROM {Statuses} )" will result in 8. Note that duplicates are not removed.

The expression "SIZE ( UNIQUE ( COLLECT Status FROM {Statuses} ) )" will result in 4, as the unique filters the duplicates.